Genshin Impact bugs are more common than what some players believe.
It's well documented on YouTube how many bugs there are in Genshin Impact, and unfortunately, it's not of the insect variety. This particular bug was found on YouTube earlier this month, but it's still fascinating for Genshin Impact players to try out.
A lot of these types of glitches work because of the way map geometry works in Genshin Impact. Typically, it involves irregular geometry next to a body of water. Sometimes, a player can jump ahead to a different part of the map, while at other times the player drops to the ocean floor.
Genshin Impact YouTuber finds a bug to walk underwater, walks all the way to the secret island
As the video says, "Walk under Water Glitch | Get to the Secret Island easily," YouTuber Paimon of Time does a masterful job showing other players how to execute this bug. So, players should expect to see them get to the secret island in the first half of the video, and then expect to see an alternative way to walk underwater in the second half.
Aside from that, the video is pretty clear on how players can execute the bug. The first half is a little trickier to execute properly, but it's still perfectly doable for players who align themselves correctly. By comparison, the bug at Liyue Harbor is significantly easier to do.
As seen in the video, Paimon of Time is able to clip through the wall, swim a fair distance, and then drops off the map to arrive in a "secret island." This island isn't shown on the map, and this method is easier than waiting a few minutes of using Cryo elements to get there.
The secret island is important to a hidden quest known as Time and The Wind, which is seen in the YouTube video. Once a player arrives at the secret island, they can do whatever they want.
The Liyue Harbor method
The second method that Paimon of Time shows is easier to execute than the first one, but it isn't as close to the secret island. That said, players can still use this method to walk underwater and explore other areas within Genshin Impact.
Players can even see the Pearl Galley if they go far enough with this method. It isn't useful, and there isn't much that a player can do with this knowledge presently.
Other notes on these methods
What makes either method viable for Genshin Impact players is that they won't drown when walking underwater. Hence, there isn't a sense of urgency when trying to get to the secret island or any other location in the Genshin Impact world.